1. Field of the Invention
This invention relates to electrophotographic image forming apparatus, and more particularly to cleaning devices for removing residual toner and debris from a charge retentive surface of an image forming apparatus.
2. Description of Related Art
In electrophotographic applications such as xerography, a charge retentive surface of a photoreceptor is electrostatically charged, and exposed to a light pattern of an original image to be reproduced, to selectively discharge the photoreceptive surface in accordance therewith. The resulting pattern of charged and discharged areas on that surface form an electrostatic charge pattern (an electrostatic latent image) conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder referred to as toner. Toner is held on the image areas by the electrostatic charge on the surface. Thus, a toner image is produced in conformity with a light image of the original beam reproduced. The toner image may then be transferred to a substrate (e.g., paper), and an image affixed thereto to form a permanent record of the image to be reproduced. The process is well known, and is useful for light lens copying from an original, and printing applications from electronically generated or stored originals, where a charge surface may be discharged in a variety of ways. Ion projection devices where charge is imagewise deposited on a charge retentive substrate operates similarly.
Multi-colored electrophotographic printing is substantially identical to the foregoing process of black and white printing. However, rather than forming a single latent image on a photoreceptor, successive latent images corresponding to different colors are recorded thereon. Each single color electrostatic latent image is developed with toner of a color complementary thereto. This process is repeated in a plurality of cycles for different colored images and their respective complementary colored toner.
Each single colored toner image is transferred to the copy sheet in superimposed registration with the prior toner image. This creates a multi-layered toner image on a copy sheet. Thereafter, the multi-layered toner image is permanently affixed to the copy sheet as described above to create a color copy. The developer material (toner) may be a liquid material or powder material.
Although, a preponderance of the toner forming the image is transferred to the paper during transfer, some toner invariably remains on the charge retentive surface of the photoreceptor, it being held thereto by relatively high electrostatic and/or mechanical forces. Additionally, paper fibers, toner additives, kaolins and other debris have a tendency to be attracted to the charge retentive surface. It is essential for optimal imaging that the toner and debris remaining on the surface be cleaned thoroughly therefrom.
The quality of images produced by such equipment depends significantly on the ability to clean the photoconductive surface before it is reused.
Blade cleaning is a highly desirable method for removal of residual toner and debris (hereinafter, collectively referred to as "toner") from a photoreceptor. In a typical application, a relatively thin elastomeric blade member is provided and supported rigidly or in an extended manner adjacent to and transversely across the photoreceptor surface with the blade edge chiseling (doctor mode) or wiping (wiper mode) toner from the surface. Subsequent to release of toner to the surface, the released toner accumulating adjacent to the blade is transported away from the blade area by a toner transport arrangement, or by gravity.
The design and development of a photoreceptor cleaner using an elastomeric blade can be simplified by the use of a rigid blade holder. This blade holder supports the blade down to its tip by eliminating the extension of the blade from the holder. An advantage of this type of holder is that the blade material can be specified such that the tip properties can be optimized for cleaning without the need for concern about the beam properties which are needed to support a conventional blade using an extension. The rigid holder also has the advantage of expanding the blade cleaning operating range for loads and blade tip angles. Because no appreciable deflection of the blade occurs, the blade tip angle can be set to within manufacturing and assembly tolerances (known in the art and peculiar to each machine), while the blade load can be set to any desired value independently of the blade tip angle. This flexibility allows combinations of tip angles and blade loads to be used which are not possible with a single extended blade cleaner. In order to achieve the desired flexibility without using a rigid holder, changes in the blade stiffness would be required either in thickness, material or extension length.
Of course, it is further understood by one skilled in the art, that use of a rigid blade holder cleaner is most effective in concert with a compliant belt photoreceptor, in order to ensure proper cleaning of the imaging surface. This is because the rigid blade and rigid photoreceptor interface could never be straight enough or aligned well enough in production to ensure either adequate contact or minimum cleaning pressure along the full length of the blade. A belt photoreceptor, therefore, supplies sufficient compliance for the photoreceptor to conform to the variations in the rigid blade in much the same way that the extended blade supplies the compliance to conform to a rigid drum photoreceptor.
During testing, however, when rigid blade holder cleaners were tested on machines with compliant belt photoreceptors, poor cleaning was observed in the center region of the blade. Initial inspection indicated that the two support rolls nearest the cleaning blade were straight and aligned well with the blade. Upon further inspection, however, it was discovered that the belt tension across the belt was non-uniform and low in the center. The lower non-uniform belt tension across the flat photoreceptor caused the cleaning load on the blade to be below the minimum acceptable cleaning load in the center of the blade. Additionally, it was further determined that the lower belt tension was caused by deflection of the adjacent support rolls which, though slight, was enough to cause the sag in belt tension in the center of the belt and thus the poor cleaning results.
Accordingly, a need exists for an apparatus to eliminate the non-uniform belt tension under rigid blade cleaners due to photoreceptor support roll deflection. This may be achieved by uniformly increasing the diameter of the Low Lateral Force (LLF) photoreceptor support rolls, however, this adds significant cost to the rolls and, therefore, the cost of the machine. Additionally, as known in the art, smaller rolls have routinely been used to improve the copier's paper stripping capability after transfer. The smaller the shaft (at the center of the LLF roll), the more diameter is available for longer rubber LLF petals. As the petals become shorter, they become stiffer and must be made thinner to compensate. Thinner petals result in more cuts and a higher rejection rate thus, higher cost.